Structural panel and method of manufacture

ABSTRACT

A fibrous core material is disposed between and bonded to metal skins to form a structural laminate having comparable strength to steel sheets of greater weight.

[0001] This application is a continuation-in-part of U.S. patentapplication Ser. No. 8/947,030, filed Oct. 8, 1997, entitled STRUCTURALPANEL WITH KRAFT PAPER CORE BETWEEN METAL SKINS, which claims thebenefit of U.S. Provisional Application No. 601038,816, filed Feb. 10,1997, and a continuation-in-part of U.S. patent application Ser. No.09/342,690, filed Jun. 29, 1999, entitled CARGO VEHICLE WALL.

TECHNICAL FIELD

[0002] The present invention relates to laminate structural panels and,more specifically, to lightweight laminates having desirable structuralcharacteristics.

BACKGROUND OF THE INVENTION

[0003] Sheet steel is used extensively to form panels. The requiredstructural characteristics, such as stiffness, vary depending upon thespecific application. When higher stiffness values are required, thesteel thickness is typically increased. Increasing sheet steelthickness, however, produces a panel which is not only heavier, but alsomore expensive.

[0004] A number of approaches have been taken in the past to provideimproved structural characteristics of panels, without substantiallyincreasing weight or materials cost. For example, composites of steelsheets having a solid polymer core have been used in applications wheresound deadening and vibration dampers are required. The specificstiffness of polymer core products, however, is less than desirable.

[0005] In copending U.S. patent application Ser. No. 08/947,030, filedOct. 8, 1997, entitled STRUCTURAL PANEL WITH KRAFT PAPER CORE BETWEENMETAL SKINS, the entire disclosure of which is incorporated herein byreference, structural panels are disclosed which are laminate structureshaving metals skins separated by and bonded to an intervening layer ofpaper. The laminates described therein have high specific stiffness.

[0006] In copending U.S. patent application Ser. No. 09/342,690, filedJun. 29, 1999, entitled CARGO VEHICLE WALL, the entire disclosure ofwhich is incorporated by reference, truck trailers having walls formedof laminates of metal skins and paper are described. The truck wallsprovide good structural characteristics while still minimizing weightand cost.

SUMMARY OF THE INVENTION

[0007] In one aspect a structural laminate is provided having first andsecond skins of sheet metal. Each of the sheet metal skins has athickness of at least about 0.005 inches. A fibrous core layer isprovided between the sheet metal skins and is bonded to the skins. Inone aspect, the fibrous core layer is impregnated with an adhesive resinwhich bonds the core layer directly to the skins. In another aspectlayers of adhesive are placed between the core material and the skinswhich bonds the core to the skins. The resulting laminate structure isextremely lightweight compared to a single steel sheet of comparablethickness and strength.

[0008] In one aspect, the fibrous core is paper and the metal skins aregalvanized steel to provide corrosion resistance.

[0009] In still another aspect, the fibrous core layer has a pluralityof channels extending between the metal skins. These transverse channelsare filled with adhesive to create adhesive bridges that further bondthe skins to the core.

[0010] In still another aspect of the invention, a method of forming astructural laminate is provided. The method includes the steps ofplacing a fibrous core material between two metal skins and applyingpressure to the trilaminate to promote bonding between the core and themetal skins. In one aspect a number of trilaminates are prepared whichare then stacked one upon another and are then pressed to simultaneouslybond the individual layers.

[0011] In still another aspect, the present invention providesnon-planar laminates and a method of making non-planar laminates. Themethod includes the steps of forming a laminate by placing a fibrouscore between the metal skins, providing a means for bonding the core tothe skins and forming the laminate into a non-planar object using metalforming techniques. In one aspect the metal forming technique utilizedis a die press. In one aspect, heat is applied to the laminate eitherbefore, after or during the die press operation.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 is a cross-section of the laminate of the presentinvention.

[0013]FIG. 2 is a cross-section of a laminate of the present invention,illustrating the use of adhesive channels.

[0014]FIG. 3 is a diagram depicting a press for bonding the individuallayers into the inventive laminate.

[0015]FIG. 4 is a cross-section of the non-planar laminate of theinvention.

[0016]FIG. 5 is a diagram depicting a die press forming the non-planarlaminate of FIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION

[0017] Referring now to FIG. 1 of the drawings, panel 20 is shown havingmetal skins 22 and intervening fibrous core 24. It will be appreciatedthat panel 20 is essentially a trilaminate structure having threedistinct layers which cooperate to form a functional unit. Theproperties of this functional unit are superior to that of itsindividual elements as will be more fully described herein.

[0018] One preferred construction of panels 20 in which paper forms thefibrous core is fully described in the aforementioned U.S. patentapplication Ser. No. 08/947,030 filed Oct. 8, 1997.

[0019] Referring again to FIG. 1 of the drawings, in one preferredconstruction, skins 22 are most preferably formed of zinc coated steel,with each layer 22 having a thickness of about 0.005 inch or greater,preferably from between about 0.005 inch and about 0.030 inch and morepreferably from about 0.005 inch to about 0.012 inch in thickness. Inone embodiment intervening fibrous layer 24 preferably has a thicknessof from about 0.01 inch and greater and preferably from about 0.01 inchto about 0.05 inch. Thus, the overall thickness of panel 20 in oneembodiment will typically be between about 0.020 inch and about 0.110inch. A panel having the dimensions set forth above in the descriptionof FIG. 1 of the drawings and having the preferred layer thicknessesjust described will typically have weight that is about 40 -70% of theweight of a single sheet of steel of comparable dimensions andstiffness.

[0020] The individual layers shown in FIG. 1 will now be described inmore detail. As stated, metal skins 22 generally will be flat havingplanar surfaces on each side. Metals which may be utilized to form skins22 are preferably selected from the group consisting of steel, aluminum,copper alloys and various combinations thereof. Metals which provideadequate structural and (if required) corrosion resistance properties inthe specific environment in which panel 20 is used, at the lowest cost,are most desirable. Most preferable is zinc coated sheet steel.

[0021] As will be appreciated by those skilled in the art, steel comesin a number of grades based on the amount of carbon and other elementswhich it contains. Broadly these grades can be described as low carbonsteel, medium steel and high carbon steel. Preferred for use herein arelow carbon steel and low carbon micro-alloyed high-strength steel(HSLA). The most preferred metal skins for use in the present inventionare cold rolled steel, galvanized steel, tin coated steel, and stainlesssteel. It may be desirable to utilize single sided galvanized sheet witha galvanized surface comprising the outer surface of skins 22 with theinner surfaces of the skins being bare metal for adhesion purposes. Inone embodiment differential zinc coating is preferred, i.e. a light zinccoating on the inside surface and a heavier zinc coating on the outersurface. In one embodiment, zinc coated steel is cold rolled with zincon the surface to the final thickness.

[0022] Layer 24 is a fibrous material. Although not wishing to be boundby any theory, it is believed that the fibrous nature of layer 24 maycontribute to the desirable structural characteristics of panel 20,including resistance to delamination. As used herein, the term“fibrous,” without limiting its scope in the context of the invention,is intended to mean a generally homogeneous collection of fibers, eithernatural or synthetic, which can be formed into a sheet product.

[0023] The most preferred fibrous material for use herein as layer 24, amaterial which is believed to be unique among fibrous sheets, is paper.As will be appreciated by those skilled in the art, paper is essentiallya matted or felted structure of fibrous material formed into arelatively thin sheet through the medium of a dilute suspension of pulpand water. It is composed essentially of cellulose fibers. Pulp forpaper making can be prepared by grinding wood or other plant mattermechanically, by chemical processing (sulfite, kraft, or soda) and alsoby chemically treating cotton, linen and hemp rags, waste, straw, andthe like.

[0024] In the present invention, paper formed using the kraft process ismost preferred. It will be appreciated by those skilled in the art thatthe (raft process (which may also be referred to as sulphate pulping oralkaline process) results in papers of high physical strength and bulk.One preferred paper is sold as saturating kraft paper, by Westvaco, ofCharleston, S.C.

[0025] Also, as will be appreciated by those skilled in the art, theaverage alignment of cellulose fibers in paper is controlled somewhat bythe “machine direction” during production of the paper. It is believedthat in the present invention the orientation of the paper in thelaminates is a factor which may affect the stiffness and strength of thelaminate. Most preferred are laminates where the machine direction ofthe kraft paper is a line parallel to a neutral axis of bending of thelaminate.

[0026] Another class of fibrous materials for use herein isplastic-fiber paper. Unlike paper, plastic-fiber paper is a collectionof synthetic (i.e., synthetic polymer) fibers formed into a sheet. Forexample, it can be made from 100 percent high-density polyethylenefibers by spinning very fine fibers and then bonding them together withheat and pressure. Nylon paper, such as Nomex type 410, is produced fromshort fibers (floc) and smaller binder particles (fibrids) of ahigh-temperature-resistant polyamide polymer, formed into a sheetproduct.

[0027] In one preferred embodiment of the invention, layer 24 isprovided as a resin-impregnated fibrous material. Where layer 24 iskraft paper, the paper is saturated with a resin which is then dried.Most preferred for use herein is phenolic resin-impregnated kraft paper.Polyester resin impregnation may also be suitable in some applications.Methods of impregnating paper with resin will be well-known to thoseskilled in the art. In essence, the preferred resin-impregnated paper isformed by immersing a substrate paper web in liquid phenolic resin.Typically, layers of saturated impregnated paper are layered together toform a single layer of semi-cured impregnated paper. One method ofproducing resin-impregnated paper is described in Canadian Patent No.2,203,200 which issued Oct. 22, 1997. Other methods of impregnationinclude coating and spreading the resin on the paper. Although it may besuitable or desirable in specific applications to go beyond the rangesset forth hereinafter with respect to the resin content layer 24, in apreferred embodiment of the present invention resin constitutes fromabout 15% to about 45% by weight of resin-impregnated layer 24.

[0028] In most instances thermosetting resins are preferred for use inimpregnating paper layer 24, although in some applications thermoplasticresins may be acceptable. In the case of thermosetting resins, as statedthe resin will generally be cured to B-stage prior to forming panel 20,but it may be possible to fully cure the impregnated paper prior to thelaminate pressing operation (controlled heat and pressure) describedbelow. In the case of a phenolic resin, the resin is cured to B-stageprior to lamination. It is then fully cured as skins 22 and impregnatedpaper core 24 are laminated together using the press. It may be suitablein some applications to include a number of standard additives in theresin such as curing agents, fillers and the like.

[0029] With or without resin impregnation of fibrous layer 24, it may bedesirable or required to use a layer of adhesive to bond skins 22 tofibrous core 24. A number of adhesives may be suitable in specificapplications, including epoxies, phenolics, isocyanates, polyurethanes,and hot-melts. A particularly preferred adhesive for this purpose is anitrile phenolic sold as “Arofene 1166” from Ashland Chemical. Theadhesive may be applied directly to layer 24 or to metal skins 22 orboth by any number of methods. It is preferred to pretreat the steelwith a conversion coating such as a complex oxide or zinc phosphate toimprove bond integrity and corrosion resistance.

[0030] In another embodiment, and referring now to FIG. 2 of thedrawings, fibrous layer 24 of laminate 30′ has a plurality of holes 26extending therethrough. In some applications, holes 26 provide adhesive“bridges” as more fully described in the aforereferenced U.S. patentapplication Ser. No. 08/947,030.

[0031] Referring now to FIG. 3 of the drawings, one method of assemblingskins 22 and layer 24 is shown using press 28. Press 28 includes platens30 which move towards one another in the customary manner usinghydraulics or the like. Platens 30 are preferably heatable so both heatand pressure can be applied to the laminates to cure the resin and bondthe adhesive. More specifically, in FIG. 3 two metal/fiber/metal panels32 and 34 are shown stacked upon each other. As stated above, thephenolic resin in layer 24 is at the B stage of cure prior to the pressoperation and heated platens 30 complete the cure of the phenolic resinduring the pressing process. Also it is to be understood that while asingle layer 24 is shown in the drawings, typically several sheets offibrous material will be stacked on top of each other to build layer 24up to the preferred thickness. In some applications it may be desirableto place a layer of adhesive between each sheet forming layer 24.

[0032] Referring again to FIG. 3 of the drawings, after a number ofmetal/fiber/metal laminates are stacked (preferably from 5-20), thepress closes to apply pressure to the stack. The times, temperatures andpressures will vary widely depending upon thicknesses of skins 22 andlayers 24, types of resin used to impregnate the central layer, and thetype of adhesive used, if any, between the central layer and the metalskins. Most preferably, the temperature used should be adequate to curethe phenolic resin fully and assure bonding of the adhesive layers, ifany. For phenolic resin impregnated paper the temperature is preferablybetween about 300° F. and 350° F. in a low pressure press, 25 to 400 psiand more preferably, 25 to 100 psi. The time required with thesepreferred temperatures and pressures in order to form securely bondedlaminates typically will be between 15 and 60 minutes. That is, heat andpressure will be applied typically for a period within this range. Afterthe pressed stack has cooled, it is removed from the press, individualpanels are then separated.

[0033] In some applications, a sizing agent may be added to layer 24 toimprove its resistance to water and to enhance interfiber bonding. Flameretardants such as those based on combinations of bromine, chlorine,antimony, boron and phosphorous may also be added to layer 24.

[0034] Turning now to FIG. 4 of the drawings, the method of formingnon-planar laminate objects in accordance with the present inventionwill be explained. It will be appreciated that in some applications, thelaminate panels of the present invention may substitute for a metalsection which is not a flat sheet. Therefore, in one embodiment of thepresent invention provides a non-planar laminate 36. As used herein, theterm “nonplanar” shall mean a geometry other than a flat sheet (as isthe laminate panel prior to the metal forming operation). Non-planarlaminate 36 is shown for illustration purposes as a hat-shaped sectionhaving metal skins 38 bonded to opposite sides of fibrous core layer 40.The individual layers and their means of assembly into a trilaminatesuitable for serving as a blank in the metal forming operation are thesame as described above in connection with FIGS. 1 and 2 of thedrawings.

[0035] In FIG. 5 of the drawings a simple die press 42 is shown havingmale (44) and female (46) die components. A planar metal/fiber/metalblank is placed on the female half of the die. The male die portion ofthe press is then used to form non-planar laminate 36 by closing press42. In some instances, the individual layers (metal/fiber/metal) will bepreassembled in the laminated state and used as a blank. In otherinstances, it may be suitable to form a stack of unbonded or partiallybonded layers (i.e., metal/fiber/metal) which are then simultaneouslybonded together or subsequently bonded together and formed to shape asthey are pressed (with or without heat). It may also be suitable topartially bond the layers together with an adhesive having sufficienttack strength and then fully bond the laminate in the paint bake cycle.

[0036] While particular embodiments of this invention are shown anddescribed herein, it will be understood, of course, that the inventionis not to be limited thereto since many modifications may be made,particularly by those skilled in this art, in light of this disclosure.It is contemplated, therefore, by the appended claims, to cover any suchmodifications as fall within the true spirit and scope of thisinvention.

I claim:
 1. A structural laminate comprising: first and second skins ofsheet metal, each of said skins having a thickness of at least about0.005 in; a fibrous core disposed between said skins of sheet metal; andsaid fibrous core being bonded to said skins of sheet metal.
 2. Thestructural laminate recited in claim 1, wherein said sheet metal isselected from the group consisting of cold rolled steel, galvanizedsteel, tin-coated steel and stainless steel.
 3. The structural laminaterecited in claim 1, wherein said fibrous core is adhesively bonded tosaid skins of sheet metal.
 4. The structural laminate recited in claim1, wherein each of said skins has a thickness of from about 0.005 in. toabout 0.030 in.
 5. The structural laminate recited in claim 1, whereinsaid fibrous core is impregnated with a resin.
 6. The structurallaminate recited in claim 1, wherein said fibrous core has a thicknessof at least about 0.01 in.
 7. The structural laminate recited in claim1, wherein said fibrous core has a thickness of from about 0.01 in. and0.05 in.
 8. The structural laminate recited in claim 1, wherein saidlaminate is a structural panel.
 9. The structural laminate recited inclaim 1, further including layers of adhesive disposed between saidfibrous core and each of said skins.
 10. The structural laminate recitedin claim 1, wherein said fibrous core is paper.
 11. The structurallaminate recited in claim 1, wherein said fibrous core is a syntheticpolymer.
 12. The structural laminate recited in claim 1, furtherincluding a plurality of channels extending through said fibrous coreand extending between said metal skins.
 13. The structural laminaterecited in claim 12, wherein said channels are filled with adhesive toform adhesive bridges between said metal skins.
 14. The structurallaminate recited in claim 1, wherein said sheet metal skins are zinccoated steel which has been cold rolled with zinc on the surface. 15.The structural laminate recited in claim 1, wherein said fibrous core isa plurality of webs of fiber adhesively bonded to each other.
 16. Thestructural laminate recited in claim 1, wherein said laminate isnon-planar.
 17. The structural laminate recited in claim 1, wherein saidmetal skins are steel which has been pretreated with a conversioncoating to promote bond integrity and corrosion resistance.
 18. Thestructural laminate recited in claim 1, wherein said metal skins areformed of low carbon micro-alloyed high-strength steel.
 19. Thestructural laminate recited in claim 1, further including a flameretardant in said fibrous core.
 20. A method of forming a structurallaminate comprising: providing first and second skins of sheet metal,each of said skins having a thickness of at least about 0.005 in.;providing a fibrous core; and adhesively bonding said fibrous core tosaid skins such that said fibrous core is disposed between said skins.21. A method of forming a non-planar laminate comprising the steps of:providing first and second skins of sheet metal, each of said skinshaving a thickness of at least about 0.005 in.; providing a fibrouscore; placing said fibrous core between said skins and in contacttherewith thereby creating a laminate structure; and metal forming saidlaminate structure to form said non-planar laminate.